As being self-luminescent, organic EL devices have excellent visibility. They can be driven at a voltage as low as several volts to tens of volts so that the weight of the device including a driving circuit can be reduced. Such characteristics make it possible to use them as a thin film display, illumination and back light. Rich color variation is also one characteristic of organic EL devices.
In particular, organic EL devices are promising as displays to be mounted on vehicles owing to their wide viewing angle, high contrast and low-temperature operation. Their industrialization has however been impeded by severe quality standards for vehicles because they have problems peculiar to them such as lowering in brightness and low heat resistance.
For improving the brightness lifetime, a method of using a light emitter layer formed by mixing a hole transporting material and an electron transporting material as a host and a light emitting additive as a dopant has conventionally been proposed (refer to JP 8-48656A).
According to the investigation by the present inventors, however, it has been confirmed that some materials employed as the hole transporting material have not enough heat resistance and by the exposure to high temperature circumstances, for example, at 100° C., dark spots appear clearly, resulting in lowered brightness.
Various hole injecting and/or transporting materials have conventionally been disclosed as a host of a light emitter layer (refer to JP 2000-156290A, International Publication No. 98/8360 and JP 8-48974A). Some of them are materials having a high glass transition point for improving high-temperature durability. More specifically, materials having, in the molecule thereof, at least three triphenylamines in order to have a glass transition point of 100° C. or greater are proposed.